1. Field of the Invention
The present invention relates to an apparatus for determining the attitude of an artificial satellite on the basis of whether stars caught by a star sensor are identified as those in a catalog of stars on the entire celestial sphere.
2. Description of the Related Art
FIGS. 5A, 5B, and 5C show the operation and configuration of a star identification section of a conventional artificial satellite attitude determination apparatus which is, for example, the one disclosed in Japanese Patent Application Laid-open No. 61-6100.
The conventional artificial satellite attitude determination apparatus disclosed in the above-mentioned publication performs identification of stars as described below. As shown in FIG. 5A, an artificial satellite 21 carrying a star sensor observes a plurality of stars 23 on the celestial sphere by means of the star sensor while traveling around the earth 22. In correspondence with the maximum field of view of the star sensor, a circle 25 is made about an observed star 24 among all stars on the celestial sphere observable with the star sensor with the maximum value of a star sensor viewing angle as a radius. Stars 26 found within the circle and darker than the star 24 are sorted out and lines are radially drawn from the central star 24 to the stars 26 to form a radial line pattern. The formed radial pattern is assigned a pattern number, which is the same as the star identification number of the star 24.
Thereafter, as shown in FIG. 5B, a set 27 of patterns is prepared in this manner from the stars on the entire celestial sphere, and a table 28 is formed from the set 27 of patterns by combining characteristic quantities and the pattern numbers of all the patterns. Another table 29 is formed in which the characteristic quantities and the pattern numbers are arranged in the descending order of the magnitudes of the characteristic quantities of the table 28. A function 31 of addresses in the table 29 is formed for approximations to the characteristic quantities. A pattern number table 30 is also prepared which contains only the pattern numbers in the table 29. A star identification process is as described below. A pattern is generated in the above-described manner about a star which is brightest in image data obtained through observation with the star sensor. The pattern is compared with the function 31 to extract a matching portion from a corresponding sub-table of the table 30. Extraction of only one matching portion is regarded as a success in identification.
FIG. 5C shows the configuration of an apparatus for determining the attitude of an artificial satellite on the basis of the results of determination as to whether stars caught by a star sensor are identified as those on a catalog of stars on the entire celestial sphere. Image data obtained through observation with a star sensor 1 is compared with a pattern formed on the basis of a star catalog 2 by a star identification section 3. Occurrence of only one match is recognized as a success in identification. If the result is only one match, an attitude is uniquely computed from the identification result by an attitude computation section 4, thereby completing initial attitude acquisition.
FIG. 6 is a block diagram showing the configuration of another conventional apparatus for determining the attitude of an artificial satellite. In FIG. 6, the components identical or corresponding to those of the conventional apparatus shown in FIG. 5C are indicated by the same reference numerals. The description for the corresponding components will not be repeated. As new reference numerals, reference numeral 6 indicates an attitude updating section for updating the values of attitude candidates on the basis of star images output from the star sensor 1 and a star catalog stored in the star catalog data base 2, and reference numeral 7 indicates an attitude propagation section for computing the values of an attitude candidate at the present sampling time from the values of an attitude candidate and the satellite body angular velocity at the preceding sampling time.
In the artificial satellite determination apparatus arranged as shown in FIG. 6, a star can be identified even when the number of stars in the field of view of the star sensor 1 imaged at one time is smaller than the number necessary for identification. Also, a plurality of attitude angle candidates are prepared in the star identification section 3 and the attitude computation section 4, and as star images are obtained by repeating image pickup with the star sensor 1, those impossible as a candidate for a solution are removed by the attitude updating section 6. In this apparatus, therefore, it is not necessary to devise a special method for extracting characteristic quantities in the star identification section 3.
However, the above-described conventional artificial satellite attitude determination apparatus shown in FIG. 5 is arranged to narrow corresponding candidates down to one in the star identification section 3 and to use only one solution in the attitude computation section 4 and in the other subsequent sections. To do so, it is necessary to devise a special method for extracting characteristic quantities such that comparison of only one frame of star image in the field of view of one star sensor at a time point suffices for completion of identification. Moreover, there are other restrictions such that it is necessary to increase the field of view of the star sensor 1 to such a sufficiently large extent as to always cover the necessary number of stars for identification, or identification at the time point is abandoned when the number of stars falling into the field of view is insufficient.
In the above-described conventional artificial satellite attitude determination apparatus shown in FIG. 6, the process of narrowing down candidates by removing impossible ones requires a considerably long time for narrowing down to one candidate.
In view of the above-described circumstances, an object of the present invention is to provide an artificial satellite attitude determination apparatus capable of narrowing attitude candidates down to a correct one in a short time.
In order to attain the above-mentioned object, according to one aspect of the present invention, there is provided an apparatus for determining the attitude of an artificial satellite comprising one or a plurality of star sensors, a star catalog data base in which information on each of stars in an inertial coordinate system is stored, a star identification section in which star images output from the star sensors are collated with a star catalog in the star catalog data base with respect to each star sensor to output a group of corresponding candidates, an attitude computation section for computing a value of an attitude candidate of the artificial satellite with respect to each corresponding candidate, an attitude updating section for updating the value of the attitude candidate on the basis of star images output from the star sensor and the star catalog, an attitude propagation section for computing the value of the attitude candidate at the present sampling time from the value of the attitude candidate at a preceding sampling time and the artificial satellite body angular velocity, and an attitude candidate unification section for comparing the values of a plurality of attitude candidates to combine the same into one, the attitude candidate unification section being provided in a loop formed by the attitude updating section and the attitude propagation section.
Also, the attitude candidate unification section compares the value of the attitude candidate computed by the attitude computation section at the present sampling time and the value of the attitude candidate obtained by propagating the attitude candidate obtained by being computed in the past and by being propagated to the present sampling time by using the satellite body angular velocity at the corresponding sampling time to combine the attitude candidates into one.
Further, the attitude candidate unification section compares the attitude candidate computed from one of the star sensors with the attitude candidate computed from another of the star sensors to combine the attitude candidates into one.
Furthermore, the attitude updating section predicts star images on the star sensor screen and also star images on the other star sensor screen with respect to each of the attitude candidates on the basis of the star catalog, verifies the star images by comparison with star image data output from the corresponding star sensor, and eliminates those in the attitude candidates apparently different from the star image data.